diff --git a/src/__fixtures__/json.ts b/src/__fixtures__/json.ts
index 12edfab3..ca869f5a 100644
--- a/src/__fixtures__/json.ts
+++ b/src/__fixtures__/json.ts
@@ -671,6 +671,325 @@ export const JSON_RPC_RESPONSE_FIXTURES = {
   ],
 };
 
+export const JSON_RPC_PENDING_RESPONSE_FIXTURES = {
+  valid: [
+    ...JSON_RPC_SUCCESS_FIXTURES.valid,
+    ...JSON_RPC_FAILURE_FIXTURES.valid,
+    {
+      id: 1,
+      jsonrpc: '2.0',
+    },
+    {
+      id: 1,
+      jsonrpc: '2.0',
+      error: undefined,
+    },
+    {
+      id: 1,
+      jsonrpc: '2.0',
+      result: undefined,
+    },
+    {
+      id: 1,
+      jsonrpc: '2.0',
+      result: undefined,
+      error: undefined,
+    },
+    {
+      id: 1,
+      jsonrpc: '2.0',
+      result: {
+        foo: 'bar',
+      },
+      error: {
+        code: -32000,
+        message: 'Internal error',
+      },
+    },
+  ],
+  invalid: [
+    {},
+    [],
+    true,
+    false,
+    null,
+    undefined,
+    1,
+    'foo',
+    {
+      jsonrpc: '2.0',
+      error: {
+        code: -32000,
+        message: 'Internal error',
+      },
+    },
+    {
+      id: 1,
+      error: {
+        code: -32000,
+        message: 'Internal error',
+      },
+    },
+    {
+      id: {},
+      jsonrpc: '2.0',
+      error: {
+        code: -32000,
+        message: 'Internal error',
+      },
+    },
+    {
+      id: [],
+      jsonrpc: '2.0',
+      error: {
+        code: -32000,
+        message: 'Internal error',
+      },
+    },
+    {
+      id: true,
+      jsonrpc: '2.0',
+      error: {
+        code: -32000,
+        message: 'Internal error',
+      },
+    },
+    {
+      id: false,
+      jsonrpc: '2.0',
+      error: {
+        code: -32000,
+        message: 'Internal error',
+      },
+    },
+    {
+      id: undefined,
+      jsonrpc: '2.0',
+      error: {
+        code: -32000,
+        message: 'Internal error',
+      },
+    },
+    {
+      id: 1,
+      jsonrpc: '1.0',
+      error: {
+        code: -32000,
+        message: 'Internal error',
+      },
+    },
+    {
+      id: 1,
+      jsonrpc: 2.0,
+      error: {
+        code: -32000,
+        message: 'Internal error',
+      },
+    },
+    {
+      id: 1,
+      jsonrpc: {},
+      error: {
+        code: -32000,
+        message: 'Internal error',
+      },
+    },
+    {
+      id: 1,
+      jsonrpc: [],
+      error: {
+        code: -32000,
+        message: 'Internal error',
+      },
+    },
+    {
+      id: 1,
+      jsonrpc: true,
+      error: {
+        code: -32000,
+        message: 'Internal error',
+      },
+    },
+    {
+      id: 1,
+      jsonrpc: false,
+      error: {
+        code: -32000,
+        message: 'Internal error',
+      },
+    },
+    {
+      id: 1,
+      jsonrpc: null,
+      error: {
+        code: -32000,
+        message: 'Internal error',
+      },
+    },
+    {
+      id: 1,
+      jsonrpc: undefined,
+      error: {
+        code: -32000,
+        message: 'Internal error',
+      },
+    },
+    {
+      id: 1,
+      jsonrpc: '2.0',
+      error: {
+        code: -32000,
+      },
+    },
+    {
+      id: 1,
+      jsonrpc: '2.0',
+      error: {
+        message: 'Internal error',
+      },
+    },
+    {
+      id: 1,
+      jsonrpc: '2.0',
+      error: [],
+    },
+    {
+      id: 1,
+      jsonrpc: '2.0',
+      error: {},
+    },
+    {
+      id: 1,
+      jsonrpc: '2.0',
+      error: true,
+    },
+    {
+      id: 1,
+      jsonrpc: '2.0',
+      error: false,
+    },
+    {
+      id: 1,
+      jsonrpc: '2.0',
+      error: null,
+    },
+    {
+      id: 1,
+      jsonrpc: '2.0',
+      error: 'foo',
+    },
+    {
+      id: 1,
+      jsonrpc: '2.0',
+      error: 1,
+    },
+    {
+      id: 1,
+      jsonrpc: '2.0',
+      error: {
+        code: {},
+        message: 'Internal error',
+      },
+    },
+    {
+      id: 1,
+      jsonrpc: '2.0',
+      error: {
+        code: [],
+        message: 'Internal error',
+      },
+    },
+    {
+      id: 1,
+      jsonrpc: '2.0',
+      error: {
+        code: true,
+        message: 'Internal error',
+      },
+    },
+    {
+      id: 1,
+      jsonrpc: '2.0',
+      error: {
+        code: false,
+        message: 'Internal error',
+      },
+    },
+    {
+      id: 1,
+      jsonrpc: '2.0',
+      error: {
+        code: null,
+        message: 'Internal error',
+      },
+    },
+    {
+      id: 1,
+      jsonrpc: '2.0',
+      error: {
+        code: undefined,
+        message: 'Internal error',
+      },
+    },
+    {
+      id: 1,
+      jsonrpc: '2.0',
+      error: {
+        code: 'foo',
+        message: 'Internal error',
+      },
+    },
+    {
+      id: 1,
+      jsonrpc: '2.0',
+      error: {
+        code: -32000,
+        message: {},
+      },
+    },
+    {
+      id: 1,
+      jsonrpc: '2.0',
+      error: {
+        code: -32000,
+        message: [],
+      },
+    },
+    {
+      id: 1,
+      jsonrpc: '2.0',
+      error: {
+        code: -32000,
+        message: true,
+      },
+    },
+    {
+      id: 1,
+      jsonrpc: '2.0',
+      error: {
+        code: -32000,
+        message: false,
+      },
+    },
+    {
+      id: 1,
+      jsonrpc: '2.0',
+      error: {
+        code: -32000,
+        message: null,
+      },
+    },
+    {
+      id: 1,
+      jsonrpc: '2.0',
+      error: {
+        code: -32000,
+        message: undefined,
+      },
+    },
+  ],
+};
+
 export const COMPLEX_OBJECT = {
   data: {
     account: {
diff --git a/src/json.test.ts b/src/json.test.ts
index a0917763..05bd02e8 100644
--- a/src/json.test.ts
+++ b/src/json.test.ts
@@ -6,6 +6,7 @@ import {
   JSON_FIXTURES,
   JSON_RPC_FAILURE_FIXTURES,
   JSON_RPC_NOTIFICATION_FIXTURES,
+  JSON_RPC_PENDING_RESPONSE_FIXTURES,
   JSON_RPC_REQUEST_FIXTURES,
   JSON_RPC_RESPONSE_FIXTURES,
   JSON_RPC_SUCCESS_FIXTURES,
@@ -18,12 +19,14 @@ import {
   assertIsJsonRpcRequest,
   assertIsJsonRpcResponse,
   assertIsJsonRpcSuccess,
+  assertIsPendingJsonRpcResponse,
   getJsonRpcIdValidator,
   isJsonRpcFailure,
   isJsonRpcNotification,
   isJsonRpcRequest,
   isJsonRpcResponse,
   isJsonRpcSuccess,
+  isPendingJsonRpcResponse,
   isValidJson,
   validateJsonAndGetSize,
 } from '.';
@@ -254,6 +257,50 @@ describe('json', () => {
     });
   });
 
+  describe('isPendingJsonRpcResponse', () => {
+    it.each(JSON_RPC_PENDING_RESPONSE_FIXTURES.valid)(
+      'returns true for a valid pending JSON-RPC response',
+      (response) => {
+        expect(isPendingJsonRpcResponse(response)).toBe(true);
+      },
+    );
+
+    it.each(JSON_RPC_PENDING_RESPONSE_FIXTURES.invalid)(
+      'returns false for an invalid pending JSON-RPC response',
+      (response) => {
+        expect(isPendingJsonRpcResponse(response)).toBe(false);
+      },
+    );
+  });
+
+  describe('assertIsPendingJsonRpcResponse', () => {
+    it.each(JSON_RPC_PENDING_RESPONSE_FIXTURES.valid)(
+      'does not throw for a valid pending JSON-RPC response',
+      (response) => {
+        expect(() => assertIsPendingJsonRpcResponse(response)).not.toThrow();
+      },
+    );
+
+    it.each(JSON_RPC_PENDING_RESPONSE_FIXTURES.invalid)(
+      'throws for an invalid pending JSON-RPC response',
+      (response) => {
+        expect(() => assertIsPendingJsonRpcResponse(response)).toThrow(
+          'Not a pending JSON-RPC response',
+        );
+      },
+    );
+
+    it('includes the value thrown in the message if it is not an error', () => {
+      jest.spyOn(superstructModule, 'assert').mockImplementation(() => {
+        throw 'oops';
+      });
+
+      expect(() =>
+        assertIsPendingJsonRpcResponse(JSON_RPC_FAILURE_FIXTURES.invalid[0]),
+      ).toThrow('Not a pending JSON-RPC response: oops');
+    });
+  });
+
   describe('isJsonRpcResponse', () => {
     it.each(JSON_RPC_RESPONSE_FIXTURES.valid)(
       'returns true for a valid JSON-RPC response',
diff --git a/src/json.ts b/src/json.ts
index 2680142b..f1aaaff9 100644
--- a/src/json.ts
+++ b/src/json.ts
@@ -162,8 +162,8 @@ export type JsonRpcNotification<Params extends JsonRpcParams> = InferWithParams<
 >;
 
 /**
- * Type guard to narrow a JSON-RPC request or notification object to a
- * notification.
+ * Type guard to narrow a {@link JsonRpcRequest} or
+ * {@link JsonRpcNotification} object to a {@link JsonRpcNotification}.
  *
  * @param requestOrNotification - The JSON-RPC request or notification to check.
  * @returns Whether the specified JSON-RPC message is a notification.
@@ -175,8 +175,8 @@ export function isJsonRpcNotification(
 }
 
 /**
- * Assertion type guard to narrow a JSON-RPC request or notification object to a
- * notification.
+ * Assertion type guard to narrow a {@link JsonRpcRequest} or
+ * {@link JsonRpcNotification} object to a {@link JsonRpcNotification}.
  *
  * @param requestOrNotification - The JSON-RPC request or notification to check.
  */
@@ -192,7 +192,8 @@ export function assertIsJsonRpcNotification(
 }
 
 /**
- * Type guard to narrow a JSON-RPC request or notification object to a request.
+ * Type guard to narrow a {@link JsonRpcRequest} or @link JsonRpcNotification}
+ * object to a {@link JsonRpcRequest}.
  *
  * @param requestOrNotification - The JSON-RPC request or notification to check.
  * @returns Whether the specified JSON-RPC message is a request.
@@ -204,8 +205,8 @@ export function isJsonRpcRequest(
 }
 
 /**
- * Assertion type guard to narrow a JSON-RPC request or notification object to a
- * request.
+ * Assertion type guard to narrow a {@link JsonRpcRequest} or
+ * {@link JsonRpcNotification} object to a {@link JsonRpcRequest}.
  *
  * @param requestOrNotification - The JSON-RPC request or notification to check.
  */
@@ -220,6 +221,23 @@ export function assertIsJsonRpcRequest(
   }
 }
 
+export const PendingJsonRpcResponseStruct = object({
+  id: JsonRpcIdStruct,
+  jsonrpc: JsonRpcVersionStruct,
+  result: optional(unknown()),
+  error: optional(JsonRpcErrorStruct),
+});
+
+/**
+ * A JSON-RPC response object that has not yet been resolved.
+ */
+export type PendingJsonRpcResponse<Result extends Json> = Omit<
+  Infer<typeof PendingJsonRpcResponseStruct>,
+  'result'
+> & {
+  result?: Result;
+};
+
 export const JsonRpcSuccessStruct = object({
   id: JsonRpcIdStruct,
   jsonrpc: JsonRpcVersionStruct,
@@ -263,7 +281,38 @@ export type JsonRpcResponse<Result extends Json> =
   | JsonRpcFailure;
 
 /**
- * Type guard to check if a value is a JsonRpcResponse.
+ * Type guard to check whether specified JSON-RPC response is a
+ * {@link PendingJsonRpcResponse}.
+ *
+ * @param response - The JSON-RPC response to check.
+ * @returns Whether the specified JSON-RPC response is pending.
+ */
+export function isPendingJsonRpcResponse(
+  response: unknown,
+): response is PendingJsonRpcResponse<Json> {
+  return is(response, PendingJsonRpcResponseStruct);
+}
+
+/**
+ * Assert that the specified JSON-RPC response is a
+ * {@link PendingJsonRpcResponse}.
+ *
+ * @param response - The JSON-RPC response to check.
+ * @throws If the specified JSON-RPC response is not pending.
+ */
+export function assertIsPendingJsonRpcResponse(
+  response: unknown,
+): asserts response is PendingJsonRpcResponse<Json> {
+  try {
+    assert(response, PendingJsonRpcResponseStruct);
+  } catch (error) {
+    const message = isErrorWithMessage(error) ? error.message : error;
+    throw new Error(`Not a pending JSON-RPC response: ${message}.`);
+  }
+}
+
+/**
+ * Type guard to check if a value is a {@link JsonRpcResponse}.
  *
  * @param response - The object to check.
  * @returns Whether the object is a JsonRpcResponse.
@@ -275,7 +324,7 @@ export function isJsonRpcResponse(
 }
 
 /**
- * Type assertion to check if a value is a JsonRpcResponse.
+ * Type assertion to check if a value is a {@link JsonRpcResponse}.
  *
  * @param response - The response to check.
  */
@@ -291,7 +340,8 @@ export function assertIsJsonRpcResponse(
 }
 
 /**
- * Type guard to narrow a JsonRpcResponse object to a success (or failure).
+ * Type guard to narrow a {@link JsonRpcResponse} object to a success
+ * (or failure).
  *
  * @param response - The response object to check.
  * @returns Whether the response object is a success.
@@ -303,7 +353,8 @@ export function isJsonRpcSuccess(
 }
 
 /**
- * Type assertion to narrow a JsonRpcResponse object to a success (or failure).
+ * Type assertion to narrow a {@link JsonRpcResponse} object to a success
+ * (or failure).
  *
  * @param response - The response object to check.
  */
@@ -319,7 +370,8 @@ export function assertIsJsonRpcSuccess(
 }
 
 /**
- * Type guard to narrow a JsonRpcResponse object to a failure (or success).
+ * Type guard to narrow a {@link JsonRpcResponse} object to a failure
+ * (or success).
  *
  * @param response - The response object to check.
  * @returns Whether the response object is a failure, i.e. has an `error`
@@ -332,7 +384,8 @@ export function isJsonRpcFailure(
 }
 
 /**
- * Type assertion to narrow a JsonRpcResponse object to a failure (or success).
+ * Type assertion to narrow a {@link JsonRpcResponse} object to a failure
+ * (or success).
  *
  * @param response - The response object to check.
  */